1. Description of the Prior Art
The invention relates to image projectors in which the orthogonal and complementary directions of polarization of light are modulated in order to form an image to be projected.
In image projectors, notably video image projectors, the image projected on a screen results from a spatial modulation of light. The light produced by a source is guided up to a light modulation assembly that groups together the means required to achieve the spatial modulation of light. It is common practice to use a liquid crystal display (LCD) matrix screen or panel, comprising a matrix array of elementary liquid crystal cells, as a spatial light modulator. The liquid crystal cells are controlled through a video signal and each cell represents a pixel. The LCD matrix screen is thus an electro-optical slide.
2. Description of the Prior Art
The simplest way of forming an image by means of a liquid crystal display matrix screen is to illuminate it by a linearly polarized light. To this effect, it is common practice, firstly, to interpose a polarizer between the light source and the matrix screen, in order to select a particular direction of polarization for the the illumination of the matrix screen and, secondly, after the matrix screen, to position a second polarizer (which is then called a "polarization analyzer") which converts the angular modulation of polarization produced by the matrix screen into modulation of intensity. The major drawback of this method is that by the selection of a particular direction of polarization for the illumination of the LCD matrix screen more than half of the light energy is lost.
A known way of improving the efficiency of illumination of the LCD matrix screen is to separate or split the light spatially into two polarized beams having complementary directions of polarization, with a view to using all the light produced by the source. To this effect, several methods may be used:
(a) According to a first method (see European patent application No. 0.372.905), each polarized beam illuminates a different LCD matrix screen, and the images formed separately by the two LCD matrix screens are superimposed. The major drawback of this method is that it requires the use of two LCD screens per primary color, which leads to a very high cost of the image projector;
(b) another method consists of making the direction of polarization of one of the two polarized beams rotate by 90.degree. so that the direction of polarization is the same in both polarized beams, and in then directing the two beams so as to illuminate the LCD matrix screen. A structure such as this is shown and described in the report (page 90) of the "EURODISPLAY" conference organized by the S.I.D. in Amsterdam in 1990.
Irrespectively of whether it is the method (a) or the method (b), described above, that is used, another major cause of the low luminous efficiency lies in the losses of light due to the differences between the shape of the LCD matrix screen and the shape of the section of the non-polarized light beam produced by the light source.
Thus, for example, in the context of the development of high-definition, wide-screen television sets, the image is provided with a 16/9 format (ratio of length to width). An image projector meeting this definition uses a spatial modulator screen having this format. In this case, if we assume, as in the most common example, that the non-polarized light beam produced by the souce has a circular section, the fraction of light energy used to illuminate the rectangle that constitutes the spatial modulator screen corresponds only to the ratio SR/SF of the surface area SR of this rectangle to the surface area SF of the section of the non-polarized light beam, namely 0.54. This corresponds to the optimum configuration, namely when the surface SR of the modulator screen has the 16/9 format and is as large as possible and is at the same time contained in the surface area SF of the section of the non-polarized light beam.
The present invention is aimed chiefly at reducing the losses of light due to the differences between the shape of the spatial modulator screen and the shape of the section of the non-polarized light beam, in image projectors using the two orthogonal and complementary directions of polarization of light. The invention can be applied both to monochromatic image projectors and to color image projectors.